Method and system of link control

ABSTRACT

The present invention relates to retransmissions in a communications system and more especially it relates to link load control in a cellular mobile radio system particularly in a Universal Mobile Telecommunications System, UMTS. BLER targets are set depending on load.

This application is the US national phase of international applicationPCT/SE02/00693 filed 6 Apr. 2002, which designated the US.PCT/SE02/00693 claims priority to SE Application No. 0101281-4 filed 06Apr. 2001. The entire contents of these applications are incorporatedherein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to retransmissions in a communicationssystem, and more especially it relates to link load control in acellular mobile radio system, particularly to a Universal MobileTelecommunications System, UMTS or WCDMA system.

BACKGROUND AND DESCRIPTION OF RELATED ART

Retransmission of data to or from a mobile station, MS, or userequipment, UE, is previously known. If data is received in error, it isrequested for retransmission. If errors are frequent, data is requestedfor retransmission frequently. When the relative amount of transmissionsthat are retransmissions of previously transmitted data increases theeffective user data rate reduces, for a constant channel data rate.

Increasing transmission power can often reduce transmission errors.Consequently, increasing transmission power can reduce the relativeamount of retransmissions. Correspondingly, transmission power can bereduced if the relative amount of transmission errors is sufficientlysmall.

U.S. Pat. No. 5,461,639 describes forward link power control in a CDMAsystem where error statistics are collected. The error rate is comparedto a threshold. If the error rate exceeds the threshold, transmissionpower is increased. A code selector determines at which redundancy levelto transmit. For each level of redundancy, the selector maintains apredetermined target error rate.

International Patent Application WO0013362 describes a method and devicefor adapting a transmission data rate or a transmitter power to thetransmission quality of a transmission channel.

A channel data rate that gives an acceptable error rate at a givensignal to noise ratio is selected. Transmission power is controlled.

It is also known to use medium access control and radio link controllayers of a UMTS protocol structure in acknowledged mode for dedicatedchannels and to broadcast system information.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.331v3.5.0, France, December 2000, specifies an RRC protocol. Section 8.1.1describes broadcast of system information.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.322v3.5.0, France, December 2000, specifies the RLC protocol. The RLC layerprovides acknowledged data transfer service.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.301v3.6.0, France, September 2000, specifies in chapter 5 Radio InterfaceProtocol Architecture of a UMTS system.

None of the cited documents describes a method and system of linkcontrol where error targets are selected as a function of transmissionload in an ARQ scheme of a radio communications system.

BRIEF SUMMARY

In a power and interference limited system transmission power cannot beincrease beyond all limits. For a high system traffic load, availablepower for each user is more restricted than at a low system trafficload.

Perceived quality to an individual user needs to be balanced to overallsystem performance. Using only one fixed block error rate target, or oneor more block error rate targets independent of traffic load, results ina too low perceived quality at low traffic load and a reduced throughputdue to traffic being barred during high traffic load.

Consequently, an object of this invention is to increase perceivedquality and reduce delay as perceived by a user during low traffic load.

It is also an object to increase system throughput during high trafficload.

A further object is to present a system and method of communicating loadcontrol commands and block error rate targets between various systemelements.

Finally, it is an object to adaptively control link load using doubleloops including link load estimates and targets for transmission powercontrol.

These objects are met by a method and system of retransmissions suchthat target block error rate is determined in relation to traffic load.

A first embodiment meets the objects for uplink load control with loadmeasurements in a base station, BS, or Node B and load control in aradio network controller, RNC, communicating transmission power controltargets, determined from block error rate targets, to the BS/Node B tobe used for uplink transmission power control of a user equipment, UE.The block error rate targets, BLER targets, can be either determined tobe the same for all connections or determined individually on a link bylink basis.

A second embodiment meets the objects for downlink load control withload measurements in a BS/Node B and load control in an RNC bycommunicating a set of BLER targets to the user equipment and dependingon the load situation, which BLER target to use within the set.

According to a third embodiment for downlink load control, only BLERtargets to be considered by the UE are communicated to it and not anentire set of BLER targets.

As with uplink BLER targets, downlink BLER targets can be determinedindividually for each connection, if more than one, of a UE or only oneBLER targets is determined for all one or more connections of a UE.

The invention is particularly well suited for high-speed TCP connectionsinterconnecting the Internet and a user equipment.

Preferred embodiments of the invention, by way of examples, aredescribed in detail with reference to the accompanying drawings below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays throughput versus channel data rate for an ARQ system,according to the invention.

FIG. 2 shows target block error rates versus load, according to theinvention.

FIG. 3 schematically illustrates uplink control, according to theinvention.

FIG. 4 shows a block diagram of one embodiment of downlink load control,according to the invention.

FIG. 5 displays a block diagram of another embodiment of downlink loadcontrol, according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Wireless systems, in particular, reach their highest system throughputwhen a total amount of transmitted packets partly includesretransmissions of previously transmitted packets. In FIG. 1, thehighest rate with which packets are transmitted on the channel, thepacket transmission rate, and a corresponding retransmission rate areschematically depicted versus raw channel bit rate, the gross rate. Thepacket transmission rate increases linearly with the gross rate. As thegross rate increases, transmission errors will be more frequent and therelative amount of retransmissions increases, everything else beingequal. As the retransmission rate approaches the transmission rate forsuccessively higher gross rates, the throughput approaches zero.According to the schematic and exemplary diagram of FIG. 1, there is amaximum throughput when approximately one third of all transmissions areretransmissions. Preferably, it is less than one third in a runningsystem. FIG. 1 only illustrates an exemplary situation.

In general, the fraction of retransmissions for which an optimumthroughput is achieved depends on various parameters. One of theseparameters is the block error rate, BLER. With reference to FIG. 1, asraw channel bit rate (the abscissa) increases transmitted bit energydecreases and, as a consequence, bits transmitted at a high raw bitrate, the gross rate, are more error prone. Therefore, a block or apacket being composed of a number of bits will also be more error proneas the gross rate increases. At the receiving end errors are detected bymeans of error detecting codes, well known in the art.

When automatic repeat request, ARQ, is made use of, as is the case in aWCDMA system, a packet or block detected to be in error is requested forretransmission. This request could be in the form of a negativeacknowledgement, when an error is detected, or an omitted positiveacknowledgement. The greater the fraction of packets detected to be inerror, the higher the retransmission rate.

As can be seen from the simplified diagram in FIG. 1, when less than themaximum throughput, the same net rate (throughput) is achieved for morethan one gross rate. At the lower gross rate the relative amount ofretransmissions is inferior to the relative amount of retransmissions atthe higher gross rate.

When retransmissions become a substantial amount of all transmissions,the users will perceive a low quality of service due to increasedaverage delays. There will also be a greater delay variance as eachpacket may require multiple retransmissions to get through withouterrors. For short instances the user might then imagine the connectionbeing broken.

A WCDMA system will be designed to interact with e.g. the Internet. Mostapplications on the Internet use protocols, such as TCP (TransportControl Protocol), that controls the transmission rate, based on linkquality in terms of packet loss and delay characteristics. Consequently,besides the negative effect of retransmission delays as such onperceived quality, substantial queuing delay can also lead to secondaryeffects further reducing quality of service. An obvious solution tothese secondary effects is to develop a modified TCP less sensitive ofthe increased number of retransmissions typical of, e.g., wirelesssystems. However, this is greatly undesirable as it may introduceproblems to user applications.

At high load it is consequently a goal to operate a system close to itsmaximum throughput, to make best use of scarce resources. At low loadthe system throughput is not a scarce resource and therefore there is noreason to optimize system for maximum throughput but for perceivedquality.

FIG. 1 illustrates a situation with a received signal strength that doesnot vary, or does not vary very much, in relation to an overall noiseand interference level. A modern communication system makes use oftransmission power control to keep received signal to interference ratioclose to a constant target ratio, where the target is preset to a fixlevel.

As explained above it is advantageous to operate a system at variousretransmission rates depending on load. As packets detected to be inerror are retransmitted the retransmission rate is reflected into ablock error rate, BLER. This invention utilizes different BLER targetsfor different load levels. This is illustrated in FIG. 2. At a high loadsituation between thresholds 2 and 3, a high BLER target, BLER target 3,is used. BLER target 3 corresponds to an operational setting close tomaximum throughput of FIG. 1. At a low load situation between thresholds0 and 1, a small BLER target is used to guarantee a high user quality,where maximum system throughput is less critical. For intermediary loadsbetween thresholds 1 and 2 an intermediary BLER target, BLER target 2,is used.

One or more BLER targets are evaluated at the receiving end. If thenumber of erroneous packets exceeds the BLER target, transmission powerneeds to be increased, everything else being equal. Consequently, thereceiving end should command a transmission power increase.Correspondingly, if the BLER is less than the BLER target, transmissionpower should be commanded to decrease. BLER targets are set ordetermined in relation to traffic load as previously described inrelation to FIG. 2. BLER targets are communicated as needed for closedloop evaluation.

As described in relation to FIG. 1, the objective of transmission powercontrol is to maintain receiver signal to interference ratio, SIR, closeto constant. This constant is an SIR target. In order to achieve thedesired transmission power control, the BLER target selection affectsthe SIR target to be used for the transmission power control, i.e. BLERis controlled indirectly.

FIG. 3 illustrates an embodiment for uplink control according to theinvention. A radio network controller, RNC, determines a set of BLERtargets, e.g. BLER targets 1–3 of FIG. 2. As examples only, BLER targetsin the range of 1–30%, depending on traffic load, demonstrate highefficiency. Offered uplink load is set in the RNC in means <<UL loadmonitor>> and received BLER is achieved from uplink data transmission inmeans <<BLER estimate>>. Means <<UL load control>> determines which BLERtarget from the set of BLER targets to use by indicating by selector<<High/low target>> if BLER target should be increased or decreased. Ofcourse, alternative ways of indicating which particular BLER target fromthe set of BLER targets to select also falls within the scope of thisinvention. The module <<UL load control>>, located in the RNC, receivesinformation on the uplink load, preferably from a base station entity<<Node B>>. This can originate from measurements, e.g. Received SignalStrength Indicator, RSSI, or from other traffic observations, such ascounting number of connections. From the BLER estimate, the BLER target,optionally also including the old BLER target/old SIR target for whichthe BLER estimate is determined, a target <<SIR target>> to be used iscalculated or looked up. This is performed by the uplink outer looppower control means <<UL outer loop PC>>.

A base station <<Node b>> comprises means for uplink load measurements,as explained above. It also comprises means <<UL inner loop PC>> forinner loop uplink transmission power control and means <<SIR estimate>>for estimating uplink signal to interference ratio. Means <<UL innerloop PC>> compares <<SIR target>>, achieved from RNC, and estimated SIRon uplink, achieved from means <<SIR estimate>>, and commands a userequipment UE to increase or decrease transmission power in a poweramplifier PA depending on the outcome of the comparison. BLER and SIRwill be estimated in the RNC and <<Node B>>, respectively, on datatransmitted with the controlled transmission power, closing the innerand outer loops.

The set of BLER targets as determined by the RNC, can be determinedindividually for each connection or only one set of BLER targets isdetermined for all, or a subset of all, connections of the RNC. Also theSIR target can be determined individually for each connection or onlyone SIR target is determined for all, or a subset of all, connections ofthe Node B. In UMTS Radio Access Bearers, RABs, make available radioresources (and services) to user applications. For each mobile stationthere may be one or several RABs. Each RAB is associated with a class ofQuality of Service, QoS. When selecting individual BLER targets it ispreferred to select BLER target and, correspondingly, SIR target inaccordance with the QoS class for an RAB.

Basically, the load control as described for uplink in relation to FIG.3 is also applicable for downlink, with the obvious interchange of upand down, and <<Node B>> and UE, respectively. However, there are somedifferences due to the fact that BLER targets are determined on theRadio Access Network side, RAN side, preferably in the RNC and the loadmeasurements are likewise determined on the RAN side, preferably in the<<Node B>>. This will have repercussions on how the BLER target fordetermining SIR target is communicated to the UE.

FIGS. 4 and 5 show two embodiments of downlink load control. In FIG. 4,a complete set of BLER targets is transferred to the UE and, dependingon the downlink load control, a target selector is communicated,indicating which BLER target of the stored set of BLER targets to use.Since the set of BLER targets only need to be transferred at connectionsetup or is updated infrequently, signaling could be reduced as comparedto a solution where the BLER target to use is communicated. A blockdiagram of such a solution is illustrated in FIG. 5. If the respectiveBLER target elements of the set of BLER targets are changed frequently,or the DL load control is designed to only infrequently switch betweenthe various BLER targets of a set of BLER targets, the solutionaccording to FIG. 5 will perform more efficiently.

According to both FIGS. 4 and 5, offered downlink load is set in the RNCin means <<UL load monitor>>. The RNC determines a set of BLER targets.As for uplink load control, <<Node B>> comprises means <<DL loadmeasurement>> for determining actual downlink load. The downlink loadcan e.g. be estimated from the downlink Transmitted Carrier Power orfrom traffic observations. Means <<DL load control>> determines whetherBLER target should increase or decrease, depending on downlink loadmeasured, as described above in relation to FIG. 2. Means <<DL outerloop PC>> calculates a target signal <<SIR target>> input to means <<DLinner loop PC>> determining downlink inner loop power control. If means<<SIR estimate>> indicates a downlink SIR less than target <<SIRtarget>>, means <<DL inner loop PC>> commands the power amplifier PA of<<Node B>> to increase transmission power. If estimated downlink SIR isgreater than target <<SIR target>>, PA is commanded to decreasetransmission power. Means <<DL outer loop PC>> receives a signalindicating estimated BLER from means <<BLER estimate>>.

FIG. 4 shows an embodiment where the entire set <<BLER targets>> of BLERtargets is transferred from the RNC to the UE. For-selection of theparticular BLER target to use in selecting an SIR target also a selector<<High/low target>> is communicated to the UE. Preferably the signalcarrying selector <<High/low target>> is broadcast in a systeminformation message when the same BLER target is used for allconnections. In the UE, means <<DL outer loop PC>> determines which BLERtarget to use from the set of BLER targets as indicated by the selector<<High/low target>>. This BLER target is further compared with theestimated downlink BLER. If estimated BLER is less than the selectedBLER target, the SIR target is reduced. If estimated BLER is greaterthan the selected BLER target, the SIR target is increased.

The embodiment of FIG. 5 transfers a selected BLER target from RNC to UEvia <<Node B>>. In the RNC a target signal <<BLER target>> is determinedin means <<DL BLER control>>. Target signals <<High/low target>> and<<BLER targets>>, determined as described above, are input to means <<DLBLER control>>. This means for downlink BLER control selects aparticular BLER target from the set of BLER targets accordingly. In theUE the BLER target carried by signal <<BLER target>> is input to means<<DL outer loop PC>> for comparison with estimated BLER from means<<BLER estimate>>. Depending on the outcome of this comparison, an SIRtarget is determined as described above.

A person skilled in the art readily understands that the receiver andtransmitter properties of a <<Node B>> or a UE are general in nature.The use of concepts such as <<Node B>>, UE or RNC within this patentapplication is not intended to limit the invention only to devicesassociated with these acronyms. It concerns all devices operatingcorrespondingly, or being obvious to adapt thereto by a person skilledin the art, in relation to the invention. As two explicit non-exclusiveexamples, the invention relates to mobile stations without a subscriberidentity module, SIM, as well as user equipment including one or moreSIMs and physical entities, base stations, as well as the logical nodes,Nodes B, they represent. Further, protocols and layers are referred toin relation to UMTS terminology. However, this does not excludeapplicability of the invention in other systems with other protocols andlayers of similar functionality.

The invention is not intended to be limited only to the embodimentsdescribed in detail above. Changes and modifications may be made withoutdeparting from the invention. It covers all modifications within thescope of the following claims.

1. A method of link load control in a communications system usingautomatic repeat request, the method characterized in that transmissionpower is controlled including determining at least two targets, thefirst target being related to a target received signal strength,interference level or noise level, and the second target being relatedto a block error rate, and in that one or more signals carryinginformation on a set of second targets are transmitted to the userequipment from a radio access network.
 2. The method according to claim1 characterized in that the second target is selected from the set ofsecond targets related to block error rates.
 3. The method according toclaim 1 characterized in that the second target is selected depending ontraffic load.
 4. The method according to claim 1 characterized in thatthe first target is determined in relation to the second target.
 5. Themethod according to claim 1 characterized in that for uplink loadcontrol a network element determines the first target depending on thedifference between the selected second target and an actual value,corresponding to the second target, estimated from uplink datatransmissions.
 6. The method according to claim 1 characterized in thatfor downlink load control a user equipment determines the first targetdepending on the difference between the selected second target and anactual value, corresponding to the second target, estimated fromdownlink data transmissions.
 7. The method according to claim 1characterized in that the load control is closed loop control.
 8. Themethod according to claim 1 characterized in that the first target istarget received signal strength, interference level, noise level, signalto interference ratio, signal to noise ratio, or a combination thereof.9. The method according to claim 1 characterized in that the secondtarget is target received block error rate or retransmission rate. 10.Radio communication system comprising means for carrying out the methodin claim
 1. 11. A method of link load control in a communications systemusing automatic repeat request, the method characterized in thattransmission power is controlled including determining at least twotargets, the first target being related to a target received signalstrength, interference level or noise level, and the second target beingrelated to a block error rate, and in that one or more signals carryingselector information for selection of a second target from a set ofsecond targets are transmitted to the user equipment from a radio accessnetwork.
 12. A method of link load control in a communications systemusing automatic repeat request, the method characterized in thattransmission power is controlled including determining at least twotargets, the first target being related to a target received signalstrength, interference level or noise level, and the second target beingrelated to a block error rate, and in that one or more signals carryinga selected second target from a set of second targets are transmitted tothe user equipment from a radio access network.
 13. A method of linkload control in a communications system using automatic repeat request,the method characterized: in that transmission power is controlledincluding determining at least two targets, the first target beingrelated to a target received signal strength, interference level ornoise level, and the second target being related to a block error rate;in that for downlink load control a user equipment determines the firsttarget depending on the difference between the selected second targetand an actual value, corresponding to the second target, estimated fromdownlink data transmission; and in that one or more signals arebroadcast.
 14. A first network element in a communication system usingautomatic repeat request, the first network element comprising: meansfor link load control including means for determining at least twotargets, the first target being related to a target received signalstrength and the second target being related to a block error rate;means for uplink load monitor of offered traffic load; means forreception of first measurement data; means for comparing offered trafficload and first measurement data; means for delivering a second targetselector signal depending on the outcome of the comparison of theoffered traffic load and the first measurement data; and means fordetermining the first target depending on the relation between thesecond target and the second measurement data.
 15. The first networkelement according to claim 14 further comprising means for selecting asecond target out of a set of second targets by means of a second targetselector signal.
 16. The first network element according to claim 14further comprising means for comparing the selected second target andsecond measurement data.
 17. The first network element according toclaim 14 further comprising means for transmitting one or more signalscarrying the first target to a second network element.
 18. The firstnetwork element according to claim 14 wherein the first measurement datareflects actual traffic load.
 19. The first network element according toclaim 14 wherein the first measurement data is received signal strengthor transmitted power.
 20. The first network element according to claim14 wherein the second measurement data is block error rate orretransmission rate.
 21. The first network element according to claim 14wherein the third measurement data is signal strength, interferencelevel, noise, level signal to interference ratio, signal to noise ratio,or a combination thereof.
 22. The first network element according toclaim 14 wherein the first target is target received signal strength,interference level, noise, level signal to interference ratio, signal tonoise ratio, or a combination thereof.
 23. The first network elementaccording to claim 14 wherein the second target is target received blockerror rate or retransmission rate.